Optical Systems including Light-Guide Optical Elements with Two-Dimensional Expansion

ABSTRACT

An optical system including a light-guide optical element (LOE) with first and second sets (204, 206) of mutually-parallel, partially-reflecting surfaces at different orientations. Both sets of partially-reflecting surfaces are located between parallel major external surfaces. A third set of at least partially-reflecting surfaces (202), deployed at the coupling-in region, receive image illumination injected from a projector (2) with an optical aperture having a first in-plane width and direct the image illumination via reflection of at least part of the image illumination at the third set of at least partially-reflective facets towards the first set of partially-reflective facets with an effective optical aperture having a second width larger than the first width.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to optical systems and, in particular, itconcerns an optical system including a light-guide optical element (LOE)for achieving optical aperture expansion. Many near-eye display systemsinclude a transparent light-guide optical element (LOE) or “waveguide”placed before the eye of the user, which conveys an image within the LOEby internal reflection and then couples out the image by a suitableoutput coupling mechanism towards the eye of the user. The outputcoupling mechanism may be based on embedded partial reflectors or“facets”, or may employ a diffractive pattern. The description belowwill refer primarily to a facet-based coupling-out arrangement, but itshould be appreciated that various features of the invention are alsoapplicable to diffractive arrangements.

Two-dimensional aperture expansion within a waveguide employing internalorthogonal facets was described in FIG. 13 of U.S. Pat. No. 6,829,095B2, which is reproduced here as FIG. 1A. Reference numerals referring tothe prior art drawings are presented here in parentheses. Light fromprojector (20) propagates within the waveguide and is reflected byfacets (22a)-(22c) towards facets (23), which couple the light outtowards an observer.

PCT publication WO 2019/142177 A1 discloses a similar concept employingnon-orthogonal facets. FIGS. 2 and 29 of the PCT publication arereproduced here as FIGS. 1B and 1C, respectively. The first set offacets, here denoted (32), are non-orthogonal, so only one mode ofpropagation is reflected. The two configurations illustrated differ asto whether the regions containing the two sets of facets are overlapping(FIG. 1B) or non-overlapping (FIG. 1C).

SUMMARY OF THE INVENTION

The present invention is an optical system.

According to the teachings of an embodiment of the present inventionthere is provided, an optical system for directing image illuminationinjected at a coupling-in region towards a user for viewing, the opticalsystem comprising a light-guide optical element (LOE) formed fromtransparent material, the LOE comprising: (a) a first region containinga first set of planar, mutually-parallel, partially-reflecting surfaceshaving a first orientation; (b) a second region containing a second setof planar, mutually-parallel, partially-reflecting surfaces having asecond orientation non-parallel to the first orientation; (c) a set ofmutually-parallel major external surfaces, the major external surfacesextending across the first and second regions such that both the firstset of partially-reflecting surfaces and the second set ofpartially-reflecting surfaces are located between the major externalsurfaces, wherein the second set of partially-reflecting surfaces are atan oblique angle to the major external surfaces so that a part of imageillumination propagating within the LOE by internal reflection at themajor external surfaces from the first region into the second region iscoupled out of the LOE towards the user, and wherein the first set ofpartially-reflecting surfaces are oriented so that a part of imageillumination propagating within the LOE by internal reflection at themajor external surfaces from the coupling-in region is deflected towardsthe second region, wherein the optical system further comprises a thirdset of planar, mutually-parallel, at least partially-reflecting surfacesdeployed at the coupling-in region, the third set of at leastpartially-reflecting surfaces being deployed to receive imageillumination injected from a projector with an optical aperture having afirst width measured parallel to the major external surfaces and todirect the image illumination via reflection of at least part of theimage illumination at the third set of at least partially-reflectivefacets towards the first set of partially-reflective facets with aneffective optical aperture having a second width measured parallel tothe major external surfaces, the second width being larger than thefirst width.

According to a further feature of an embodiment of the presentinvention, the third set of at least partially-reflecting surfaces has afirst sequence of successively-increasing reflectivities in an order inwhich the image illumination reaches them, and wherein the first set ofpartially-reflecting surfaces has a second sequence ofsuccessively-increasing reflectivities in an order in which the imageillumination reaches them, the second sequence starting at areflectivity smaller than a last reflectivity of the first sequence.

According to a further feature of an embodiment of the presentinvention, a last reflectivity of the first sequence ofsuccessively-increasing reflectivities is greater than 90%.

According to a further feature of an embodiment of the presentinvention, a majority of the image illumination directed towards thefirst set of partially-reflecting surfaces undergoes exactly onereflection from the third set of at least partially-reflecting surfaces.

According to a further feature of an embodiment of the presentinvention, a majority of the image illumination directed towards thefirst set of partially-reflecting surfaces undergoes two reflectionsfrom the third set of at least partially-reflecting surfaces.

According to a further feature of an embodiment of the presentinvention, the third set of at least partially-reflecting surfaces areintegrated as part of the LOE and located between the major externalsurfaces.

According to a further feature of an embodiment of the presentinvention, the third set of at least partially-reflecting surfaces areparallel to the first set of partially-reflecting surfaces.

According to a further feature of an embodiment of the presentinvention, the third set of at least partially-reflecting surfaces arenon-parallel to the first set of partially-reflecting surfaces.

According to a further feature of an embodiment of the presentinvention, an inter-surface spacing of the third set of at leastpartially-reflecting surfaces is smaller than an inter-surface spacingof the first set of partially-reflecting surfaces.

According to a further feature of an embodiment of the presentinvention, a surface area of each at least partially-reflecting surfaceof the third set of at least partially-reflecting surfaces is smallerthan a surface area of each partially-reflecting surface of the firstset of partially-reflecting surfaces.

According to a further feature of an embodiment of the presentinvention, the first region and the second region are non-overlapping.

According to a further feature of an embodiment of the presentinvention, there is also provided an image projector for projecting acollimated image having an angular field of view about an optical axis,the image projector being optically coupled to the LOE so as tointroduce the collimated image into the LOE via the third set of atleast partially-reflecting surfaces at the coupling-in region as apropagating image propagating within the LOE by internal reflection atthe major external surfaces, the propagating image being partiallyreflected by the first set of partially-reflecting surfaces to generatea deflected propagating image propagating within the LOE by internalreflection at the major external surfaces, the deflected propagatingimage being partially reflected by the second set ofpartially-reflecting surfaces to generate a coupled-out image directedoutwards from one of the major external surfaces towards the user.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1A, discussed above, corresponds to FIG. 13 of U.S. Pat. No.6,829,095 B2;

FIGS. 1B and 1C, discussed above, correspond to FIGS. 2 and 29 of PCTPatent Application Publication No. WO 2019/142177 A1, respectively;

FIGS. 2A and 2B are schematic isometric views of an optical systemimplemented using a light-guide optical element (LOE), constructed andoperative according to the teachings of the present invention,illustrating a top-down and a side-injection configuration,respectively;

FIGS. 3A and 3B are schematic illustrations of the effects of differentspacing of partially-reflecting internal surfaces on redirection ofimage illumination from a projector with a given optical aperture widthfrom a first direction to a second direction within a substrate;

FIG. 4A is a schematic front view of a light-guide optical element (LOE)according to the teachings of an embodiment of the present invention,illustrating three-stage expansion of an optical aperture from aprojector to illumination coupled-out towards a viewer;

FIGS. 4B and 4C are schematic isometric representations of twoimplementations of the LOE of FIG. 4A using partially-reflectinginternal surfaces that are orthogonal and oblique, respectively, for thefirst two stages of aperture expansion;

FIGS. 5A and 5B are schematic front and isometric views, respectively,of a variant implementation of the LOE of FIG. 4A in whichpartially-reflecting internal surfaces for performing two stages ofoptical aperture expansion are deployed in regions which are at leastpartially overlapping;

FIG. 6 is a schematic representation in angular space (polarcoordinates) of the relative directions of the image illuminationthrough various stages of propagation through the LOE of FIG. 4C;

FIGS. 7A and 7B are schematic front views of two further variantimplementations of the LOE of FIG. 4A illustrating options for lateralinjection of image illumination;

FIG. 8A is a schematic representation of a production sequence for theLOE of FIG. 4A;

FIG. 8B is a schematic representation of a production sequence for theLOE of FIG. 5A;

FIG. 9 is a schematic front view of a further variant implementation ofthe LOE of FIG. 4A in which the geometrical form of the LOE regions ismodified;

FIGS. 10A and 10B are schematic front and isometric views, respectively,of a further variant implementation of the LOE of FIG. 4A employing arectangular waveguide section for a preliminary stage of opticalaperture expansion;

FIGS. 11A and 11B are schematic isometric views before and afterassembly, respectively, of a further variant implementation of the LOEof FIG. 4A employing a slab with internal at least partially-reflectingfacets for a preliminary stage of optical aperture expansion withoutlight guiding by TIR; and

FIGS. 12A and 12B are schematic isometric views before and afterassembly, respectively, of a further variant implementation of the LOEof FIG. 4A employing a slab with internal at least partially-reflectingfacets for a preliminary stage of optical aperture expansion with lightguiding by surfaces non-parallel with the major surfaces of the LOE.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Certain embodiments of the present invention provide an optical systemincluding a light-guide optical element (LOE) for achieving opticalaperture expansion for the purpose of a head-up display, such as anear-eye display, which may be a virtual reality display, or morepreferably an augmented reality display.

An exemplary implementation of a device in the form of a near-eyedisplay, generally designated 10, employing an LOE 12 according to theteachings of an embodiment of the present invention, is illustratedschematically in FIGS. 2A and 2B. The near-eye display 10 employs acompact image projector (or “POD”) 14 optically coupled so as to injectan image into LOE (interchangeably referred to as a “waveguide,” a“substrate” or a “slab”) 12 within which the image light is trapped inone dimension by internal reflection at a set of mutually-parallelplanar external surfaces. The light impinges of a set ofpartially-reflecting surfaces (interchangeably referred to as “facets”)that are parallel to each other, and inclined obliquely to the directionof propagation of the image light, with each successive facet deflectinga proportion of the image light into a deflected direction, alsotrapped/guided by internal reflection within the substrate. This firstset of facets are not illustrated individually in FIGS. 2A and 2B, butare located in a first region of the LOE designated 16. This partialreflection at successive facets achieves a first dimension of opticalaperture expansion.

In a first set of preferred but non-limiting examples of the presentinvention, the aforementioned set of facets are orthogonal to the majorexternal surfaces of the substrate. In this case, both the injectedimage and its conjugate undergoing internal reflection as it propagateswithin region 16 are deflected and become conjugate images propagatingin a deflected direction. In an alternative set of preferred butnon-limiting examples, the first set of partially-reflecting surfacesare obliquely angled relative to the major external surfaces of the LOE.In the latter case, either the injected image or its conjugate forms thedesired deflected image propagating within the LOE, while the otherreflection may be minimized, for example, by employingangularly-selective coatings on the facets which render them relativelytransparent to the range of incident angles presented by the image whosereflection is not needed.

The first set of partially-reflecting surfaces deflect the imageillumination from a first direction of propagation trapped by totalinternal reflection (TIR) within the substrate to a second direction ofpropagation, also trapped by TIR within the substrate.

The deflected image illumination then passes into a second substrateregion 18, which may be implemented as an adjacent distinct substrate oras a continuation of a single substrate, in which a coupling-outarrangement (either a further set of partially reflective facets or adiffractive optical element) progressively couples out a proportion ofthe image illumination towards the eye of an observer located within aregion defined as the eye-motion box (EMB), thereby achieving a seconddimension of optical aperture expansion. The overall device may beimplemented separately for each eye, and is preferably supportedrelative to the head of a user with the each LOE 12 facing acorresponding eye of the user. In one particularly preferred option asillustrated here, a support arrangement is implemented as an eye glassesframe with sides 20 for supporting the device relative to ears of theuser. Other forms of support arrangement may also be used, including butnot limited to, head bands, visors or devices suspended from helmets.

It is a particularly preferred feature of certain embodiments of thepresent invention that the optical system further includes a third setof planar, mutually-parallel, at least partially-reflecting surfaces(“facets”) deployed at the coupling-in region. The third set of facetsare not shown individually in FIGS. 2A and 2B, but are designated byregion 15. The third set of facets are deployed to receive imageillumination injected from projector 14 with an optical aperture havinga first width measured parallel to the major external surfaces of theLOE 12 and to direct the image illumination via reflection of at leastpart of the image illumination by the facets in region 15 towards thefirst set of partially-reflective facets in region 16 with an effectiveoptical aperture having a second, larger width measured parallel to themajor external surfaces of the LOE. The significance of this apertureexpansion will be discussed further below.

The third set of facets 15 are interposed in the optical path betweenprojector 14 and first set of facets 16 at the coupling region. Thephrase “at the coupling region” is used herein to encompass both a casein which the third set of facets are incorporated into the LOE at thecoupling region and where the third set of facets are external to theLOE, with both of these options being exemplified in detail below.

Reference is made herein in the drawings and claims to an X axis whichextends horizontally (FIG. 2A) or vertically (FIG. 2B), in the generalextensional direction of the first region of the LOE, and a Y axis whichextends perpendicular thereto, i.e., vertically in FIG. 2A andhorizontally in FIG. 2B.

In very approximate terms, the first LOE, or first region 16 of LOE 12,may be considered to achieve aperture expansion in the X direction whilethe second LOE, or second region 18 of LOE 12, achieves apertureexpansion in the Y direction. It should be noted that the orientation asillustrated in FIG. 2A may be regarded as a “top-down” implementation,where the image illumination entering the main (second region) of theLOE enters from the top edge, whereas the orientation illustrated inFIG. 2B may be regarded as a “side-injection” implementation, where theaxis referred to here as the Y axis is deployed horizontally. In theremaining drawings, the various features of certain embodiments of thepresent invention will be illustrated in the context of a “top-down”orientation, similar to FIG. 2A. However, it should be appreciated thatall of those features are equally applicable to side-injectionimplementations, which also fall within the scope of the invention. Incertain cases, other intermediate orientations are also applicable, andare included within the scope of the present invention except whereexplicitly excluded.

The POD employed with the devices of the present invention is preferablyconfigured to generate a collimated image, i.e., in which the light ofeach image pixel is a parallel beam, collimated to infinity, with anangular direction corresponding to the pixel position. The imageillumination thus spans a range of angles corresponding to an angularfield of view in two dimensions.

Image projector 14 includes at least one light source, typicallydeployed to illuminate a spatial light modulator, such as an LCOS chip.The spatial light modulator modulates the projected intensity of eachpixel of the image, thereby generating an image. Alternatively, theimage projector may include a scanning arrangement, typicallyimplemented using a fast-scanning mirror, which scans illumination froma laser light source across an image plane of the projector while theintensity of the beam is varied synchronously with the motion on apixel-by-pixel basis, thereby projecting a desired intensity for eachpixel. In both cases, collimating optics are provided to generate anoutput projected image which is collimated to infinity. Some or all ofthe above components are typically arranged on surfaces of one or morepolarizing beam-splitter (PBS) cube or other prism arrangement, as iswell known in the art.

Optical coupling of image projector 14 to LOE 12 may be achieved by anysuitable optical coupling, such as for example via a coupling prism withan obliquely angled input surface, or via a reflective couplingarrangement, via a side edge and/or one of the major external surfacesof the LOE. Where third set of facets 15 is external to the LOE, thethird set of facets are preferably integrated with the coupling-inarrangement, as will be exemplified below with reference to FIGS.11A-11C, below. Further details of the coupling-in configuration are notcritical to the invention, and are shown here only schematically.

It will be appreciated that the near-eye display 10 includes variousadditional components, typically including a controller 22 for actuatingthe image projector 14, typically employing electrical power from asmall onboard battery (not shown) or some other suitable power source.It will be appreciated that controller 22 includes all necessaryelectronic components such as at least one processor or processingcircuitry to drive the image projector, all as is known in the art.

Turning now to FIGS. 3A and 3B, this illustrates schematically thegeometry of image illumination from a projector having a certain widthof optical aperture with the first set of partially-reflecting internalsurfaces. In order to obtain uniform light illumination, the width ofthe projector's aperture 100 must be such that the reflected rays fromone facet are contiguous with the reflected rays from the next facet toavoid black lines in the display. In some cases, it is desirable thatthere is sufficient overlap that each viewing direction receives areflection from two or more facets, and most preferably from a constantnumber of facets across the aperture, thereby enhancing uniformity ofthe viewed image. FIGS. 3A and 3B illustrate cases in which differentnumber of facets (102 and 109) are illuminated by a beam from projector2 with aperture width 100. The reflected light (104, 106, 110 and 108)propagates toward the other facets (not shown in this figure).Preferably a complete and constant number of facets are illuminated. InFIG. 3A the number varies between 2 and 3, while in FIG. 3B it isconstant, with two facets contributing to the output across the entireaperture. The wider the aperture 100, the more facets are illuminatedthe more uniform is the image transmitted.

For a predefined facet spacing the aperture width must be modifiedaccordingly to generate a uniform image. A large facet spacing thereforedictates use of a large aperture. Tight spacing of facets acrosswaveguide increases production complexity and cost. On the other hand,producing a large aperture projector increases projector size. Theseconflicting design considerations are reconciled according to an aspectof the present invention by performing a preliminary stage of opticalaperture expansion between the projector and what was referred to aboveas the first set of facets. This is achieved using an additional set offacets (referred to herein as the “third set of at leastpartially-reflecting internal surfaces”).

FIG. 4A shows schematically a front view of a waveguide according tothis aspect of the present invention. The aperture of projector 2 issmall. The two arrows originating from this projector represent lightrays of the edges of this aperture. The light from this projector iscoupled into waveguide section 200 having facets 202 (which are thepreliminary, additional and “third” set of facets). As the lightpropagates in this section 200, its lateral aperture dimension (“width”)in the plane of the LOE expands as it is partially redirected byreflections from successive facets 202 towards section 207 that includesfacets 204 (referred to above as the “first” set of facets). The lightreflected from facets 204 is redirected towards section 209 thatincludes facets 206 (referred to above as the “second” set of facets),to be coupled out towards the viewer.

FIG. 4B shows isometric view of FIG. 4A. Here it can be seen that thesection 200 has same width (waveguide thickness) as 207 and 209, so thatsections 200, 207 and 209 are integrated within a contiguous LOE,sandwiched between mutually-parallel external surfaces. The guidancethroughout these sections is by total internal reflection (TIR) fromthese external surfaces. The transmission of light between the sectionsis preferably without disturbance or discontinuity, and the separatinglines shown between the sections in various views (e.g., the front viewsof FIGS. 4A, 5A, 7A, 7B and 9 ) is for ease of understanding.

Facets 206 are designed to transmit scenery light, allowing the viewer adirect view of an external scene beyond the LOE, and therefore haverelatively low reflectivity, typically below 50%. In some configurationsfacets 204 are also designed to transmit scenery light, and thereforealso have relatively low reflectivity, typically below 50%. In otherconfigurations where facets 204 are not part of the “viewing area” ofthe LOE, higher reflectivities may be used. Facets 202 preferably areoutside the viewing area of the LOE and therefore do not need totransmit scenery. High reflectivity is therefore preferably used inorder to obtain high efficiency of light transmission. Preferably, thelast facet 211 in region 200 has a high reflectivity of at least 90%,and preferably 100% reflectivity. Since section 200 is not designed totransmit scenery light, it is preferably covered (not shown) so noexternal light passes through it. Alternatively, this section 200 of thewaveguide is coated with reflective coating such as silver.

In order to provide relatively uniform image illumination intensityacross the optical aperture, one or more of the sets ofpartially-reflecting surfaces, and preferably each set, most preferablyhas a sequence of successively-increasing reflectivities in an order inwhich the image illumination reaches them. By way of example, forwaveguide region 200, a sequence of 3 facets having 33%, 50% and 100%reflectivity are effective to reflect roughly a third of the incidentillumination from each successive surface. Similar for a sequence of 4facets, 25%, 33%, 50% and 100% values are effective to reflect roughly aquarter of the incident illumination from each surface. For facets whichare within a viewing area through which the viewer observes an externalscene, the reflectivity values are lower, and the proportional increasebetween facets is smaller, but the underlying concept of the increasingsequence to compensate for a lower proportion of illumination intensityremaining within the propagating image illumination remains the same.(Where the ideal reflectivity values for successive facets arerelatively close, two or more successive facets in a region of the LOEmay be implemented with the same reflectivity value as a manufacturingsimplification, but the sequence is still referred to as “successivelyincreasing” since it is monotonically increasing, to provide the aboveeffect of enhanced uniformity.) Thus, for example, facets 204 have asecond sequence of successively-increasing reflectivities in an order inwhich the image illumination reaches them, where the second sequencestarts at a reflectivity smaller than a last reflectivity of the firstsequence (of facets 204).

In the configuration of FIG. 4A, a majority of the image illuminationdirected towards facets 204 undergoes exactly one reflection from facets202. The spacing of the facets 202 is close, ensuring continuity of theimage illumination redirected towards facets 204 across an expandedeffective aperture, as illustrated by the bounding arrows shown in LOEsection 207. This allows the use of a larger spacing for facets 204,thereby reducing production complexity and costs for the larger portionof the waveguide. For, example if the facets 202 expand the aperture bya factor of 3 (using 3 facets with progressive increasing reflectivity)then facets 204 can have roughly three times the spacing comparedwithout section 200. In more general terms, the spacing of facets 204 istypically larger than the spacing of facets 202. Additionally, thesurface area of facets 202 is typically smaller than that of facets 204.As a result, only a relatively small volume of closely-spaced facetsneeds to be produced, while complexity and production costs for themajority of the LOE structure are reduced.

FIG. 4B shows facets in sections 200 and 207 to be perpendicular to themajor external surfaces of the waveguide. FIG. 4C shows an alternativeimplementation according to which the facets of both sections 200 and207 of the waveguide are at an oblique angle to the major surfaces ofthe LOE, referred to here as “twisted facets”.

FIGS. 5A and 5B are analogous to FIGS. 4A and 4C, but illustrate thatfacets 204 and 206 may optionally be implemented in at least partiallyoverlapping regions of the waveguide, in a manner analogous to thecorresponding options taught in WO 2019/142177 A1, referred to above.The input aperture expansion section 200 is preferably implemented so asto span a majority, and preferably the full thickness, of the LOE, asshown in FIG. 5B.

FIG. 6 illustrates the image reflections for the facets in angularspace. This description is for twisted facets as described in FIG. 4Cand in FIG. 5 . The light is coupled into waveguide 200 as 1930A intoone of images 6L or 6R. These two images represent back and forth TIRreflection from the major surfaces of the LOE as the image illuminationpropagates along aperture expansion section 200. Reflection by facets202 is represented as 1938 onto 4R and 4L. These are the imagespropagating by TIR along section 207. In this non-limiting butparticularly preferred configuration, facets 202 are parallel to facets204, so the reflection by facets 204 towards section 209 is also along1938 from 4R to 6L. Here 6L and 6R also represent images propagatingalong section 209. In other words, the images propagating in section 200and 209 are here the same in angular space. The reflection by facets 206within section 209 coupling out towards the observer is represented as1934 from guided image 6R to output coupled image 8.

Circles 39 represent the TIR cutoff of the waveguide and are parallel tothe plane of the waveguide. It is apparent the images 4L and 4R arediagonal to the plane of the waveguide, i.e., with the sides of therectangular image in angular space parallel and perpendicular to themajor surfaces of the substrate, while images 6L and 6R are alignedparallel to the surfaces of the waveguide. Practically it is typicallymore convenient to construct a projector 2 for parallel coupling in thanfor diagonal. As a result, coupling in through waveguide section 200contributes to simplicity of the projector implementation, and cantherefore be of advantage even via a small number of high-reflectivityfacets that do not necessarily significantly expand the effectiveoptical aperture of the projector.

Ergonomic consideration could dictate injection the image from the sideof the waveguide, as shown in FIGS. 7A and 7B. In this case, a firstfacet 210 is advantageously implemented with a high reflectivity inorder to achieve approximate uniformity between the image illuminationtransmitted by the first facet and that reflected by the subsequentfacets. For example, if only two facets exist in section 200, the firstfacet will have 50% reflectivity and the second 100%. However, if thereare four facets then the first will have 75% reflectivity (25%transmittance), the second 33%, the third 50% and the last (210) 100%.Alternatively, facet 210 may be implemented with 100% so that alltransmission into section 207 is from the subsequent facets.

The configuration presented in FIG. 7A is based on coupling in from1930B (referring to the angular space illustration of FIG. 6 ) ontofacet 210 that reflects 1938 to 6L. Further propagation is as describedbefore.

FIG. 7B shows an equivalent configuration where the facets in section200 are at an opposite orientation to enable different position of theprojector 2.

In the side-injection cases, the first facet 210 functions primarily asa coupling-in facet, and is an exception to the successively-increasingreflectivities of facets along the sequence of facets, with the“sequence” beginning from the second facet. In these cases, a majorityof the image illumination directed towards facets 204 undergoes tworeflections from facets 202.

FIG. 8A illustrates schematically a method for integrating a waveguidewith sections as described in FIGS. 4A-4C. A set of coated plates 253 isglued together to form a stack 254 and sliced 255 a to generate thefacet section required for section 207. A set of coated plates 250 isglued together to form a stack 251 and sliced diagonally to generate thefacet section required for section 209, shown as 252 a, and a third setof coated plates 256 is glued together to form a stack 257 which issliced to generate section 258 a (the facets required for section 200).The three sections are combined 260 a and glued 262 a. The glue is indexmatched to the waveguide so minimal perturbation introduced to the lightas it passes between the sections. A thin cover glass 264 is preferablyglued on both sides of the waveguide, and optionally further polished,to generate waveguide 266 a having smooth parallel TIR surfaces.

FIG. 8B shows a similar manufacturing process suitable for thearchitecture described in FIGS. 5A and 5B. Sections 252 b, 255 b and 258b are produced in the same manner as shown in FIG. 8A, but where 258 bis twice the thickness as the others. 252 b and 255 b are stacked while258B is placed from the side as shown in 260 b. The sections are gluedtogether 262 b and transparent cover glasses 264 are glued as covers,optionally with further polishing, to generate a single waveguide 266 b.

If it is desired to incorporate two overlapping sets of facets within asingle layer, this may be done according to the technique explained inthe above-referenced WO 2019/142177 A1 with reference to FIG. 11 , wherethe resulting waveguide section containing two sets of facets iscombined with the section 258 b (corresponding to the facets of section200) attached to the side prior to addition of the cover sheets.

Although shown thus far as rectangular waveguide sections, it should benoted that the shape of the sections can change according to thepropagation of the guided light. By way of one non-limiting example,depending on the geometry of the image propagation, expanding of theimage illumination within the waveguide may in some cases requirebroadening of sections 200 and 207 along the propagation path, resultingin a waveguide form as illustrated in FIG. 9 .

Although illustrated thus far as an integrated part of an LOE guided inone dimension, the preliminary stage of aperture expansion mayoptionally be implemented in various additional configurations which areunguided, guided on different axes, or guided in two dimensions, as willnow be exemplified by the non-limiting examples of FIGS. 10A-12B.

In the non-limiting example of FIGS. 10A and 10B, section 200 isimplemented as a rectangular waveguide section 270 which guides theimage illumination in two dimensions during the preliminary apertureexpansion, prior to injection of the expanded aperture imageillumination into waveguide section 107. An air gap 295 or some opticallayer emulating an air gap is preferably provided to maintain internalreflection within waveguide section 270 except where coupled out.Examples of such 2D waveguide structures may be found in U.S. Pat. No.10,133,070 and will not be described here in detail.

FIGS. 11A and 11B illustrate a further option according to which thecoupling-in aperture expansion facets are provided without guiding ofthe image illumination by TIR. In this case, facets 202 are provided ina first section 280 which is wider than the rest of the waveguide 207.In this configuration, the light in 280 is unguided and propagatesthrough 280 while expanding in both dimensions. In this configurationthe coupling into waveguide 207 is preferably achieved via a couplingprism 285. FIG. 11A shows 280 separated from 285 for clarity. The angledorientation of 280 and coupling prism 285 facilitate uniformillumination along the thickness (vertical as shown) dimension of 207.FIG. 11B shows 280 after attachment to coupling-in prism 285.

FIG. 12A shows a further variant implementation according to which thefirst stage of aperture expansion via facets 202 is provided in a firstsection 290 that is guided in one dimension that is not parallel towaveguide 207. FIG. 12B shows placement of section 290 on top of acoupling prism 285 where an air-gap 295 is provided in order to preserveTIR guidance within section 290.

In all respects other than those explicitly described here, thearrangement of first set of partially-reflecting internal surfaces 204and the second set of partially-reflecting internal surfaces 206 withina common waveguide may be implemented according to the range of optionsdescribed in parallel PCT patent application no. PCT/IB2019/157572,which is unpublished as of the filing date of this application and doesnot constitute prior art.

In all of the front views illustrated herein, the aperture expansion ofthe present invention is represented schematically by parallel arrowsindicating the span of the optical aperture for a given ray directioncorresponding to a central pixel on the optical axis of a collimatedimage. The optical axis is not actually within the X-Y plane, but ratherhas a Z-component into the page chosen such that the entire range ofangles in the depth dimension of the field of view (FOV) undergo totalinternal reflection at the major substrate surfaces. For simplicity ofpresentation, the graphic representations herein, and the descriptionthereof, relate only to the in-plane (X-Y) component of the light raypropagation directions, referred to herein as the “in-plane component”or the “component parallel to the major external surfaces of the LOE.”

As mentioned above in the context of FIG. 3B, all of the aboveprinciples can also be applied to “sideway” configurations, where animage is injected from a POD located laterally outside the viewing areaand is spread by a first set of facets vertically and then by a secondset of facets horizontally for coupling into the eye of the user. All ofthe above-described configurations and variants should be understood tobe applicable also in a side-injection configuration.

Throughout the above description, reference has been made to the X axisand the Y axis as shown, where the X axis is either horizontal orvertical, and corresponds to the first dimension of the optical apertureexpansion, and the Y axis is the other major axis corresponding to thesecond dimension of expansion. In this context, X and Y can be definedrelative to the orientation of the device when mounted on the head of auser, in an orientation which is typically defined by a supportarrangement, such as the aforementioned glasses frame of FIGS. 3A and3B.

Although the invention has been illustrated thus far in the context of apreferred but non-limiting example of a near-eye display, it should benoted that embodiments of various aspects of the invention may be usedto advantage in other application including, but not limited to, head-updisplays (HUDs). One subset of HUDs of particular interest are HUDs forvehicles.

It will be appreciated that the above descriptions are intended only toserve as examples, and that many other embodiments are possible withinthe scope of the present invention as defined in the appended claims.

What is claimed is:
 1. An optical system for directing imageillumination from an image projector towards a user for viewing, theoptical system comprising: (a) an image projector for projecting acollimated image having an angular field of view about an optical axis,said collimated image having a projector optical aperture; (b) a firstlight guide formed from transparent material, said first light guidehaving a first pair of mutually-parallel surfaces and a second pair ofmutually-parallel surfaces, said second pair of surfaces beingorthogonal to said first pair of surfaces, said image projector beingcoupled to said first light guide so as to introduce the collimatedimage from said projector optical aperture into said first light guideso as to propagate within said first light guide by four-fold internalreflection at said first and second pairs of surfaces; and (c) alight-guide optical element (LOE) formed from transparent material, saidLOE comprising: (i) a set of mutually-parallel major external surfaces,said major external surfaces extending across a first region and asecond region, (ii) a first set of planar, mutually-parallel,partially-reflecting surfaces, located in said first region of said LOEbetween said major external surfaces and having a first orientationnon-parallel to said major external surfaces, and (iii) a second set ofplanar, mutually-parallel, partially-reflecting surfaces, located insaid second region of said LOE between said major external surfaces andhaving a second orientation obliquely angled to said major externalsurfaces, wherein said first light guide includes a third set ofinternal mutually-parallel partially-reflecting surfaces deployed so asto progressively deflect said collimated image propagating within saidfirst light guide so as to be introduced into said LOE and to propagatewithin said LOE by internal reflection at said major external surfacestowards said first set of partially-reflecting surfaces, and whereinsaid first set of partially-reflecting surface are configured toprogressively deflect said collimated image propagating within said LOEso as to propagate within said LOE by internal reflection at said majorexternal surfaces towards said second set of partially-reflectingsurfaces, and wherein said second set of partially-reflecting surfacesare configured to progressively deflect said collimated image to as tocouple said collimated image out of said LOE for viewing by the user. 2.The optical system of claim 1, wherein said first pair of mutuallyparallel surfaces of said first light guide are parallel to said majorexternal surfaces of said LOE.
 3. The optical system of claim 1, whereinboth said first and second pairs of mutually parallel surfaces of saidfirst light guide are non-parallel to said major external surfaces ofsaid LOE, and wherein the collimated image deflected by said third setof partially-reflecting surfaces passes through a coupling prism beforeentering said LOE.
 4. The optical system of claim 1, wherein thecollimated image deflected by said third set of partially-reflectingsurfaces passes through an air gap before entering said LOE.
 5. Theoptical system of claim 1, wherein the collimated image deflected bysaid third set of partially-reflecting surfaces passes through anoptical layer emulating an air gap before entering said LOE.
 6. Theoptical system of claim 1, wherein said third set of at leastpartially-reflecting surfaces has a first sequence ofsuccessively-increasing reflectivities in an order in which the imageillumination reaches them, and wherein said first set ofpartially-reflecting surfaces has a second sequence ofsuccessively-increasing reflectivities in an order in which the imageillumination reaches them, said second sequence starting at areflectivity smaller than a last reflectivity of said first sequence. 7.The optical system of claim 6, wherein a last reflectivity of said firstsequence of successively-increasing reflectivities is greater than 90%.8. The optical system of claim 1, wherein said first region and saidsecond region are non-overlapping.
 9. The optical system of claim 1,wherein said image projector is coupled to said first light guide so asto introduce the collimated image from said projector optical apertureinto said first light guide such that no ray of said collimated image isparallel to said first or second pairs of surfaces or to said third setof partially-reflecting surfaces, and wherein said collimated imagepropagates within said LOE such that no ray of said collimated image isparallel to said major external surface or to said first set ofpartially-reflecting surfaces.